1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar full-color image forming apparatus of the type developing latent images each being formed on a particular image carrier with developers of different colors to thereby produce a full-color image.
2. Discussion of Background
It is a common practice with an electrophotographic image forming apparatus to uniformly charge the surface of a photoconductive element, expose the charged surface imagewise in order to form a latent image, develop the latent image to thereby form a corresponding toner image, and transfer the toner image to a paper sheet or similar recording medium. A cleaning unit removes toner left on the photoconductive element after the image transfer so as to prepare the element for the next image formation. The toner removed by the cleaning unit is collected in the cleaning unit and then discarded. This, however, forces the operator of the apparatus to discard the collected toner every time it fills up a container, brings about pollution ascribable to the discarded toner, and obstructs the efficient use of the developer.
In light of the above, Japanese Patent Laid-Open Publication No. 56-21175, for example, proposes a toner recycling mechanism applicable to an image forming apparatus of the type using toner of a single color and constructed to return the toner collected by a cleaning unit to a developing unit, so that the toner can be reused. To promote the conveyance of residual toner and to enhance the durability of toner, Japanese Patent Laid-Open Publication Nos. 1-214874 and 2-110572 teach improved configurations of toner itself. Further, Japanese Patent Laid-Open Publication No. 2-157765 discloses a method of improving the particle size distribution of toner in relation to a dry two-ingredient type developer.
There is an increasing demand for full-color image forming apparatuses in parallel with the development of computers, color facsimile apparatus, color printers and so forth. As a result, the amount of collected toner is increasing. A problem with a full-color image forming apparatus is that it uses more toner than a monocolor image forming apparatus and therefore needs a larger container for storing collected toner. This, coupled with the fact that the toner contains harmful substances, has highlighted the necessity of toner recycling more for a full-color image forming apparatus than for a monocolor image forming apparatus.
When a conventional full-color image forming apparatus is, constructed to recycle toner, it has been customary to develop latent images formed on photoconductive elements with respective developing units, transfer the resulting toner images to a paper sheet or similar recording medium one above the other, remove toner left on the photoconductive elements with respective cleaners, and then collect the removed toner in a single container. This gives rise to a problem that the collected toner of different colors are mixed together in the container and cannot be reused for color development.
To solve the above-described problem, Japanese Patent Laid-Open Publication No. 9-288397, for example, discloses an arrangement including a plurality of photoconductive elements each being assigned to a particular color and a plurality of cleaning units each cooperating with one of the photoconductive elements. Toner left on each photoconductive element after image transfer is removed independently of the other toner and collected in a particular container. While this arrangement implements toner recycling in a full-color image forming apparatus, mixture of colors occurs in practice. Experiments showed that when a toner image of particular color formed on a downstream photoconductive element is transferred to a paper sheet, a toner image transferred from an upstream photoconductive element to the paper sheet is again partly transferred to the downstream photoconductive element and then introduced into a cleaning unit together with collected toner. Such inverse toner transfer and therefore the mixture of colors is not avoidable with the conventional apparatus.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-281484 and 9-244294.
It is therefore an object of the present invention to provide a color image forming apparatus capable of insuring desirable image quality over a long period of time by recycling toner of different colors while reducing troubles ascribable to the mixture of the toner. A color image forming apparatus of the present invention sequentially transfers toner images each being formed on a particular image carrier to a recording medium one above the other to thereby form a color image, and reuses developers removed from image carriers. A plurality of developing units each are configured to store a developer implemented by a particular coloring agent for developing a latent image electrostatically formed on one of a plurality of image carriers with the developer. A plurality of cleaning units each are configured to remove the developer remaining on a particular image carrier and collect it. A plurality of toner recycling units each are configured to return the toner collected by a respective cleaning unit to a respective developing unit. The coloring agents of the developers are chargeable to the same polarity.